LAUSR

We carry out a combined experimental and numerical investigation on the population distribution between the ground state and the excited state of molecular nitrogen ions in an 800-nm laser field. Efficient population transfers between the ground state ${\mathrm{N}}_{2}^{+}(X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+},\ensuremath{\nu}=0)$ and the excited state ${\mathrm{N}}_{2}^{+}(B{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+},{\ensuremath{\nu}}^{\ensuremath{'}}=0)$ occur with a 400-nm time-delayed pulse, resulting in an enhanced fluorescence at 391 and 428 nm. The population distribution between the ground state ${\mathrm{N}}_{2}^{+}(X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+},\ensuremath{\nu}=0)$ and the excited state ${\mathrm{N}}_{2}^{+}(B{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+},{\ensuremath{\nu}}^{\ensuremath{'}}=0)$ is extracted from the fluorescence enhancement factor. The results show that the population of ${\mathrm{N}}_{2}^{+}(X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+},\ensuremath{\nu}=0)$ decreases when the laser exceeds 2.1 mJ in our experiment. Our study provides a method to check the population information of both excited and ground states experimentally in a strong-field-induced plasma of ionized nitrogen molecules.